The invention relates to translucent electromagnetic shielding panels, that is translucent panels for shielding against electromagnetic interference or eavesdropping, which incorporate silver coatings to attenuate electromagnetic radiation.
It is known to use coatings comprising thin silver layers on glass or plastics substrates to reflect infra red radiation, or to carry an electric current which heats the coated substrate, for example to demist the substrate. It has also been proposed to use the coated panes, with the silver layer electrically connected to earth all around the periphery of the pane, as an electromagnetic shielding panel to inhibit the transmission of electromagnetic radiation; in particular, to inhibit the entry of extraneous electromagnetic radiation liable to interfere with the operation of electric equipment behind the pane, and the escape of electromagnetic radiation carrying sensitive information from electrical equipment such as computers and word processors behind the pane, while allowing the passage of visible light through the pane. Such coatings are usually used for shielding against electromagnetic radiation having a frequency in the range 20 MHz to 10000 MHz.
In general, the thicker the silver layer, the greater the attenuating effect of the coating. However, as the thickness of the silver layer is increased, the visible light reflection of the coating also increases and the light transmission of the coating falls. It would be desirable to produce an electromagnetic shielding panel having a lower reflection, and preferably a higher light transmission, for a given electromagnetic shielding effect than is obtained using known silver coatings.
EP-A 0 207 181 relates to laminated safety glass with dual heat reflecting layers. It is concerned with the problem of duplicated virtual images ("ghost images") which appear when a pane carrying a highly reflecting layer is formed into a laminate. According to the invention, the problem is overcome by incorporating two heat reflecting layers into the laminate, one on each side of the interlayer. According to EP-A 0 207 181, the heat reflecting layer preferably has a thickness of at least 40 nm and is made of a material selected from gold, silver, palladium, nickel, chromium, aluminium, titanium and oxides thereof. The patent specification is particularly concerned with laminates with a thickened interlayer to prevent shattering and, while it refers to an interlayer thickness of 0.1 mm to 50 mm, the problem of "ghost images" only becomes at all significant at interlayer thicknesses of 1 mm or more and with highly reflecting thick coatings.
UK 915 518 describes the use of a plastic radiation screen behind a window, the screen carrying a coating of metal, for example gold, platinum or nickel, to control the passage of visible light while almost completely reflecting the long wave infra red radiation of 300.degree. K. In one embodiment, two identical radiation screens are stuck to one another on their metal coated sides by an adhesive permeable to light. The use of the two screens is said to provide the additional effect that radiation falling on the screen on the room side will be reflected back into the room.
It has now been found that an electromagnetic shielding panel of reduced light reflection for a given electromagnetic attenuation may be obtained by using two separate relatively thin silver layers in place of a single thicker silver layer.